As is well known, olefins, in particular ethylene and propylene, are used to produce numerous types of intermediate and end products, which are predominantly polymeric materials. Commercial production of olefins is generally carried out by thermal cracking of hydrocarbon feedstocks containing ethane, propane, liquid naphtha or mixtures thereof.
Unfortunately, due to the very high temperatures involved, these commercial olefin producing processes also yield a substantial amount of the less desired acetylenic (alkyne) impurities such as acetylene, methylacetylene and C4 alkynes, and also diolefins, in particular allenes such as propadiene, which contaminate the target olefin streams. Propylene and ethylene feeds, for example, may contain up to several weight percent of methylacetylene and propadiene (known as MAPD), when directly obtained from the cracking process.
Other typical processes to obtain olefins, such as fluid catalytic cracking, Methanol to Olefins (MTO) and Olefin Conversion Process (OCP), may also give rise to feedstocks having high and fluctuating amounts of acetylenic impurities, predominantly methylacetylene, and diolefins, in particular allenes such as propadiene.
These same olefins are subsequently catalytically converted to a multitude of polymeric products on a large scale. Various types of catalysts can be used for the polymerisation process. In particular, metallocene catalysts are becoming increasingly prevalent in industry. Unfortunately, these new generation catalysts are, as well as being much more expensive, also very sensitive. Their activities are severely limited by impurities present in the hydrocarbon or hydrogen feed. It is well known that acetylenics and allenes are extremely strong poisons for polymerisation catalysts, particularly metallocenes.
For a person skilled in the art, an obvious solution to remove acetylenic impurities and allenes from olefin-containing hydrocarbon feedstocks is by distillation, since for example, methylacetylene and propadiene have boiling points of −23° C. and −34° C., which are sufficiently different from the boiling point of olefins (propylene −47° C., ethylene −169° C.) and therefore easily removed. However, installing a distillation tower not only implies high capital costs, but also expensive operating costs, and is thus only suitable for purifying olefins on an extremely large scale.
Several other methods are known for separating unsaturated hydrocarbon impurities from hydrocarbon feedstocks. These include, for instance, cryogenic distillation, liquid adsorption, membrane separation and pressure swing adsorption in which adsorption occurs at a higher pressure than the pressure at which the adsorbent is regenerated. Liquid adsorption is a common technique for the separation of impurities and alkenes from gaseous mixtures containing molecules of similar size, e.g. nitrogen or methane. However, both techniques have disadvantages such as high capital cost and high operating expenses. For example, liquid adsorption techniques suffer from solvent loss and need a complex solvent make-up and recovery system.
Acetylenic impurities, but also diolefins e.g. allenes, are most commonly reduced in the hydrocarbon feedstock by hydrogenation in the presence of a hydrogenation catalyst and hydrogen. However, not only is the reaction highly exothermic, but also the rate of hydrogenation of olefins to paraffins is up to 100 times faster than that of acetylenes to olefins, for example, methylacetylene to propylene. In spite of significant progress over the years, this process has significant shortcomings such as the appearance of side products such as a “green oil” and propane, and deposition of carbonaceous residues and other impurities such as arsine or carbonyl sulphide, which deactivate the catalyst. Therefore, acetylene hydrogenation processes for treating liquid or liquefiable olefins and diolefins, such as allenes, typically include an oxygenation step or a “burn” step to remove the deactivating carbonaceous residues from the catalyst, followed by a hydrogen reduction step to reactivate the hydrogenation catalyst. For example, see U.S. Pat. No. 3,755,488 to Johnson at al., U.S. Pat. No. 3,792,981 to Hettick et al., U.S. Pat. No. 3,812,057 to Morgan and U.S. Pat. No. 4,425,255 to Toyoda. However, U.S. Pat. Nos. 3,912,789 and 5,332,705 state that by using selected hydrogenation catalysts containing palladium, at least partial regeneration can be accomplished using a hydrogenation step alone at high temperatures of 316° to 371° C. and in the absence of an oxygenation step. However, these are cost intensive, inefficient, unselective hydrogenation processes, not appropriate for obtaining the purity levels necessary for polymerisation, preferably down to the ppb range. Furthermore, they do not simultaneously remove the other impurities present in the propylene feed, such as carbonyl sulphide, arsine, antimony compounds such as antimony hydride, and carbon monoxide.
Beside palladium and modified palladium, copper with some additives can be used also as a catalyst for selective hydrogenation as seen in U.S. Pat. Nos. 3,912,789 and 4,440,956. Kokai JP Number 50929-1968 describes a method of purifying vinyl compounds containing up to about 10 percent by weight of acetylenic compounds. In this method, acetylenic compounds were described as being adsorbed on an adsorption agent of 1-valent and/or O-valent copper and/or silver supported on inert carrier such as delta alumina, silica or active carbon. Separations described included 1000 ppm ethyl acetylene and 1000 ppm vinyl acetylene from liquid 1,3-butadiene, 100 ppm acetylene from ethylene gas, 100 ppm methylacetylene from propylene gas, and 50 ppm phenyl acetylene from liquid styrene (vinylbenzene). Each application used fresh adsorption agent and only a short time of one hour on stream at mild conditions of temperature and pressure. Such limited applications were likely because it is well known that acetylene and these acetylene compounds react with copper and/or silver to form copper acetylide or silver acetylide. Both the acetylide of copper and silver are unstable compounds. Because they are explosive under some conditions, their possible formation presents safety problems in operation and in handling adsorbent containing such precipitates. A current commercial process employs a copper based catalyst in the presence of hydrogen.
The use of metallic nickel/nickel oxide sorbents is known to reduce the level of certain impurities. These are carbonyl sulphide, arsine, antimony compounds such as antimony hydride, and carbon monoxide using nickel/nickel oxide materials (See EP 0 308 569, GB 2162194, GB 2242199, EP 0 648 720 and EP 2 006 011). However, until now these have never been used in the presence of hydrogen and have certainly not been used as hydrogenation catalysts.
A method to reduce the content of acetylenics and diolefins (in particular allenes e.g. propadiene) in an olefin-containing hydrocarbon feedstock is needed with minimal capital investment, whilst removing other impurities from the feedstock.
It is a further aim to convert the acetylenics and diolefins (in particular allenes e.g. propadiene) selectively over the olefins contained in the hydrocarbon feedstock.
It is an aim to reduce the acetylenics and diolefins (in particular allenes e.g. propadiene) content of olefin-containing hydrocarbon feedstocks more efficiently.
It is also an aim to provide olefin-containing hydrocarbon feedstocks suitably purified for catalytic polymerization, in particular suitable for metallocene-catalysed polymerisation.